The present invention relates to a continuously variable transmission, comprising:
a primary pulley for rotating at a rotational speed;
a secondary pulley, whereby a drive belt is wrapped around said pulleys at variable radial positions; and
a transmission control means for controlling the transmission ratio between a maximum and a minimum value, whose quotient defines a transmission ratio coverage at least in relation to a rotational speed of the primary pulley.
The present invention also relates to a method for controlling the transmission ratio accordingly.
Such a continuously variable transmission is disclosed in EP-A-0 767 324. In particular it is known therefrom, to have transmission control means influence the actual transmission ratio within a ratio control area. The transmission speed ratio is known to be defined as the quotient between the bending radius of the drive belt when running around said secondary pulley and the bending radius of the drive belt running around said primary pulley. The ratio control area is defined in a so called variogram, which is a graphical representation of the primary pulley rotational, or input speed, which is usually equal to the engine speed, and the secondary pulley rotational, or output speed, which apart from the presence of reduction means may be considered to represent the vehicle speed. The currently generally adopted ratio control area in the variogram (see FIG. 2) is a limited area surrounded by a maximum input speed Nin, a maximum output speed Nout, a straight line indicating the maximum transmission torque ratio Rmax (=the lowest speed transmission ratio, i.e. Low) and a straight line indicating the minimum transmission torque ratio Rmin (=the highest speed transmission gear, i.e. overdrive Od). The actual transmission ratio can be controlled to lie within the ratio control area, whereby Rmax and Rmin, respectively Low and OD are extreme transmission ratios, whose quotient in the known continuously variable transmission defines a fixed transmission ratio coverage value. At realising a transmission design these extreme transmission ratios usually defined by determining the smallest radial positions of the drive belt attainable for both the primary and the secondary pulley such that during operation a load on the drive belt does not exceed an empirically determined maximum load value, which positions together with a desired distance between the rotational centres of the pulleys, i.e. the transmission size, determine the above-mentioned extreme transmission ratios and thus its transmission ratio coverage value. Hereby, the smallest radial position of the drive belt for the primary pulley is determined by the maximum attainable primary pulley input speed that is usually related to a maximum speed of an engine driving a load through the transmission, whereas the smallest radial position of the drive belt for the secondary pulley is determined by the maximum attainable secondary pulley output speed that is usually related to a maximum rotational speed of a load, e.g. passenger vehicle, driven by the engine through the transmission. The maximum rotational speed of the load thereby being determined by an equilibrium between a mechanical power generated by the engine and a mechanical power needed to sustain the rotational speed of the load, e.g. to overcome resistance factors such as a wind and a rolling resistance.
Within the nowadays available electronically controlled continuously variable transmissions full use of the transmission possibilities is made, whereby its application parameters however lie within the strictly bounded above-mentioned ratio control area.
It is an aim of the present invention to further extend the possibilities of a continuously variable transmission i.e. to break through the above-mentioned boundaries, in particular to extend the application and control possibilities of the prior art transmissions.
Thereto the continuously variable transmission according to the present invention is characterised in that the transmission control means are arranged for controlling said transmission ratio such that the transmission ratio coverage varies in relation to the rotational speed of the primary pulley.
Surprisingly it has been found that there is no absolute need to confine the transmission ratio control in relation to the rotational speed of the primary pulley to a transmission ratio control coverage having a fixed value. It has been found that a variable transmission ratio coverage from a point of view of the drive belt may vary and need not have a fixed value. This per se is a break through opening new and further application possibilities for continuously variable transmissions, in particular at the now extended outer limits of the control at minimum (high gear, i.e. overdrive Od) transmission speed ratio and/or maximum (low gear, i.e. Low) transmission speed ratio. This increases the area of the known ratio control area, hence extending the application possibilities and control limits of the transmission according to the invention.
It is an important aspect of the present invention that the total mechanical tension Ttotal in the drive belt, apart from other less significant tension components, is at least determined by a torque tension Tbelt related to the torque transmitted by the drive belt, a bending tension Tbend related to the amount of bending imposed on the drive belt when it runs around the primary and secondary pulleys, e.g. as quantified by the respective running radius, and a centrifugal tension Tcentr. related to centrifugal forces acting on the belt as a result of the drive belt speed and bending. It holds roughly that:
Ttotal=Tbelt+Tbend+Tcentr.
It may be noted that the mechanical tension Ttotal in the drive belt during operation must be limited to a boundary value to prevent premature failure of the belt, for example due to fatiguing in case of a metallic drive belt, such boundary value conforming to the said maximum load value.
According to an important notion of the present invention Tbend may vary, and in particular be made dependent on the combined influence of Tbelt and Tcentr. without exceeding said above mentioned boundary value of Ttotal.
As for the drive belt speed, the centrifugal tension Tcentr. depends on the cubic of the drive belt speed. Since the belt speed depends on the rotational speed of the primary pulley and the transmission ratio through the belt""s radial position at, at such lowered speeds the cubic thereof is lowered even more, and therefore that allowable amount of bending and the bending tension Tbend resulting therefrom may be increased without Ttotal exceeding said boundary value. Hence, for a relatively low rotational speed of the primary pulley, the maximum transmission speed ratio (low gear) may be increased and/or the minimum transmission speed ratio (high gear) may be decreased, by reducing the smallest radial position of the drive belt for the primary pulley and/or the secondary pulley respectively, thus widening the allowable ratio control area with respect to that at a relatively high rotational speed of the primary pulley.
As for the torque tension Tbelt, it is usually true that said torque tension increases with increasing speed of the primary pulley at least up to a certain speed of said pulley, as a result of the engine speed/torque characteristic of an engine driving said primary pulley. Because of this the allowable amount of bending of the drive belt and the bending tension Tbend resulting therefrom may be increased even further above said certain speed without the total mechanical tension Ttotal exceeding said boundary value, at least for relatively low rotational speed of the primary pulley. It is however noted that the torque transmitted is only directly dependent on the speed of the primary pulley, if there is a fixed connection between the engine and the primary pulley. The fixed connection usually exists when the transmission is in high gear, but in low gear it is common practice to adopt a slipping clutch or torque converter between engine and primary pulley, so that the engine speed may be considerably larger than the speed of the primary pulley.
It may thus be concluded that an increase in the allowable amount of bending due to a reduced amount of transmitted torque is primarily useful for reducing the minimum transmission ratio. As a consequence, at speeds lower than a primary pulley speed conforming to the maximum secondary speed attainable in high gear, the amount of bending of the drive belt may be increased and thus the smallest radial position thereof may be even smaller to break through the boundary of a fixed transmission ratio coverage to reveal said variable transmission ratio coverage having extended application possibilities.
The above is particularly relevant in case of a push-type drive belt, e.g. known from EP-A-0588416, which comprises one or more endless metallic rings, i.e. carriers, that support a number of plate like metallic elements slideably provided on the rings along its entire circumference. This type of drive belt has the advantage that torque may be transmitted by the elements pushing each other forward along the circumference of the rings, the tension in the rings required to transmit a given amount of torque is hereby greatly reduced. Although this type of drive belt has superior mechanical characteristics when compared to a pull type drive belt without said slideably provided elements, it also has the disadvantage that, due to the presence of the elements, the belt is rather heavy. Hence, the mechanical tension due to the centrifugal forge is significant even at a relatively low drive belt speed. Accordingly, the allowable amount of bending in the transmission according to the invention thus increases sharply with decreasing speed of the primary pulley, whereby the ratio coverage of a transmission provided with the push-type drive belt may be improved considerably.
The above means that the driving-off performance can be improved, in case the dimensions and strength of the generally hydraulic control part of the control means are devised accordingly, because the maximum transmission speed ratio can be increased, as long as a critical allowable primary pulley speed is not reached. In addition, a smaller minimum, or overdrive transmission speed ratio reduces the fuel consumption relative to the prior art transmissions by lowering the engine speed at a given vehicle speed.
A particular embodiment of the transmission according to the invention is characterised that the transmission control means are arranged to control said transmission ratio such that either one or both of a maximum and a minimum attainable transmission ratio varies. Advantageously at wish, either the traction characteristics at high transmission ratios, and/or the characteristics at high speeds of a vehicle provided with the continuously variable transmission may be improved by extending the prior fixed extreme transmission ratios to variable ratios.
A further embodiment of the transmission according to the invention has the characterising feature that the transmission control means are arranged to control said transmission ratio such that the transmission ratio coverage decreases in relation to an increasing rotational speed of the primary pulley.
It is an advantage of the continuously variable transmission according to the invention that in particular the extend wherein the prior art transmission ratio limits are offset may be influenced in dependence on, and at wish in conjunction with the engine or primary pulley speed, in order to comply even better with ever increasing driving and comfort demands.
A still further embodiment of the transmission according to the invention is characterised in that the variable transmission ratio coverage is controlled such that the outer limits of its maximum and/or minimum transmission ratios in a variogram are such that their course is one of: a curved (continuous) course or a (discontinuous) course showing a stepwise change, the position of which change in a variogram showing the relation between the primary pulley rotational speed and the secondary pulley rotational speed, may be related to the speed of a vehicle provided with such a transmission.
It is advantage of the transmission according to the invention that at wish the course of the newly devised extended boundaries, i.e. the new maximum and minimum transmission speed ratios, can be controlled easily.
Advantageously in still another embodiment of the transmission according to the invention, the stepwise change is positioned at a vehicle speed value of approximately 100 to 140 km/h. preferably at a vehicle speed value within the range of 110 to 130 km/h.
Preferably, in a following embodiment of the invention, the transmission control means are programmable control means. This provides addition flexibility to the course of the control of the transmission ratio.
Further preferred embodiments and related subjects of the present invention are specified in the remaining claims and sub-claims.